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Toxicity and chelae musculature of coral reef symbiont Chlorodiella nigra

Paul Gibson 2015


Chlorodiella nigra is an ebony coloured xanthid that inhabits living coral reefs, with a distribution in tropical and sub-tropical waters from Africa, through the indo-pacific and stretching east to the central pacific islands. The most notable features of this species are their spoon-tipped, or hoof shaped chelae, as well as the presence of saxitoxin within their mesodermal tissues. This toxin is rated as one of the most toxic, natural substances known to man, so much so that it was utilized by the UN as a biological weapon.

Figure 1

Physical Description

The dorsal surface of c.nigra is granular (smooth) unlike most xanthids (Peterand Clarke, 2000). The carapace lacks regional indication, however fourantero-lateral margins are present, where fusion of the teeth is absent (Peterand Clarke, 2000). Interspecific variation is noted depending on the locationof collection, specifically in regards to tooth one, which is sometimes reducedor blunt (Peter and Clarke, 2000). A distinct cover of setae is also a notablefeature of this crab (Sorokin, 1995).

They possess spoon tipped chelae, used to scrape algae from surfaces ofcoral (Shaw and Tibbetts, 2004). Like all xanthids, the tips of their chelae are black, however this is harder to distinguish in male specimens due to their darker colouration.

Figure 2


Chlorodiella nigra is described as an epifaunal carnivore, however recent studies have shown a distinct macroalgal grazing behaviour (Shaw andTibbetts, 2004). Further studies have expanded their diets on indo-pacific reefs in particular, to periphyton, detritus and meiobenthos (Sorokin, 1995). They are infaunal symbiotic crustaceans that utilise coral colonies as a temporary refuge, relocating casually between colonies of their preferred coral, Pocillopora. Chlorodiella nigra are commonly found in heterosexual pairs within coral heads (Gallil and Vanini,1990).

Species-specific literature in the ecology of this brachyuran is quite limited,however comprehensive studies of the family xanthidae are prevalent. Xanthids are said to dominate cryptofaunal biomasses over any other coral reef family, where they play a key grazing, cleaning and protective role in their coral hosts (Shields, 2001).

Figure 3

Life History and Behaviour

As symbiotic relatives of coral hosts, Chlorodiella nigra live a cryptic lifestyle, where they are unable to live as a free living brachyuran (Sorokin, 1995). Only one study exists in regards to their behavioural characteristics. This was regarding the grazing behaviour of coral reef crabs compared to estuarine intertidal crabs, looking at C.nigra as a representative of reef brachyurans (Shaw and Tibbetts, 2004). It was found that they follow the adaptive feeding hypothesis, where their feeding behavior was maximised during peak periods of photosynthesis, as it was found that they could optimise nutrient intake whilst minimising energy expenditure (Shaw and Tibbetts, 2004). This was displayed in terms of bite rate per minute, where C. nigra displayed a greater rate of chelae to mouth movement during these peak periods (Shaw and Tibbetts, 2004).

Figure 4

Anatomy and Physiology

Defining features of the family xanthidea are their spoon tipped chelaeas well as their internal toxicity. A common theme in nature is to associate toxicity with warning colouration, a characteristic that is frequently seen amongst xanthids. Chlorodiella nigra however lack these colourations, yet harbour toxins within their claws at a lethal concentration to predators that consume them (Llewellyn, 1997). Research has found that xanthids accumulate these toxins from exogenous sources, believed to be through consumption of toxic organisms and gut bacteria (Llewellyn, 1997). ‘The 3 H-STX-binding protein in the haemolymph wouldbind toxin at an early stage of intoxication, while toxin insensitive nerveswould reduce the effect of the toxins once translocated into the crab'stissues’ (Llewellyn, 1997). The particular toxin associated with this species insaxitoxin, which is listed as one of the most toxic natural substances known toman, historically harboured in CIA and UN biological weapons (Llewellyn,1997).

Apart from toxicity, the claws of C.nigra have a unique characteristic, common amongst xanthids but not brachyurans in general. This is their spoon shaped chelaetips, with flattened cups. The role of the claw morphologies are debated between grazing and mating display functions (Peter and Clarke, 2000; Longo and Diaz, 2013). For this reason, claws of two C.nigra specimens(male and female) were dissected in hope of displaying muscle types and differences between male and female musculature, as claw display is a primarily a male display characteristic (Swanson, 2013). Muscle type in crustaceans can be categorised into two distinct forms- striated and smooth (Longo and Diaz,2013). Smooth muscle has a low density of mitochondria, utilised as low energy, continuous movements, much like involuntary muscles in a vertebrate (Longo and Diaz, 2013). Striated muscle however, are mitochondrial rich and associated with powerful, short term movements, much like skeletal muscles in a vertebrate (Longo and Diaz, 2013).

No clear evidence of striation differences was evident from the dissection unfortunately, however it is possible to differentiate features of each gender’s muscle anatomy. The female claw muscle appears to be more condensed than the male’s, possibly suggesting a similarity in bite force despite a clear difference in size. It is possible that a less condensed male claw is representative of the claw size being used for show and not function,however this is an assumption which would need to be tested. Further research into the mechanics of musculature in xanthid chelae is required and this page could provide an avenue into future studies.

Figure 5
Figure 6

Evolution and Systematics

Kingdom - Animalia

Phylum - Arthropoda

Subphylum - Crustacea

Class - Malacostraca

Subclass - Eumalacostraca

Superorder - Eucarida

Order - Decapoda

Suborder - Pleocyemata

Infraorder - Bracyura

Section - Eubracyura

Superfamily - Xanthoidea

Family - Xanthidae

Subfamily - Chlorodiellinae

Genus - Chlorodiella

Species – Chlorodiella nigra

The Chlorodiella genus is distinguished from others within the family Xanthidae via adult and first stage zoea anatomical characteristics. The most defining adult features are seven freely movable abdominal segments and the arrangements of the pleopods (Stephenson, 1945). The first male pleopod is sinus and slender without long setae and the second male pleopod is short and sigmoid (Peter and Clarke, 2000). Characteristic zoea features include an antennal exopod that is ‘subequal in length to the protopod’ (Peter and Clarke, 2000). 

Biogeographic Distribution

This species boasts a wide distribution, where it has been sampled from locations as west as Africa and the red sea, expanding east to the central pacific Islands (Peter and Clarke, 2000). Their distribution coincides with tropical reef waters, with an abundance of living corals.

Figure 7

Conservation and Threats

This species has no formal threats to their populations and are thus not formally listed as threatened.

Studies into the species composition differences at bleached and unbleached coral sites revealed a distinct macrofaunal dominance of xanthid crabs at unbleached sites compared to a near absence of xanthids at bleached sites (Sorokin, 1995). The close relationship between living corals and mutualistic xanthids suggests a future threat to the family and thus Chlorodiella nigra, through the process of climate change. 


Galil, B., Vannini, M. (1990) Research on the coast of Somalia:Xanthidae, Trapeziidae, Carpilidae, Menippidea (Crustacea Brachyura). Tropical Zoology. 3, 21-56.

Lasley, R., Klaus, S., (2015). Phylogenetic relationships of the ubiquitous coral reef crab subfamilyChlorodiellinae (Decapoda, Brachyura, Xanthidae).  Zoologica Scripta44165178.

Llewellyn, L. (1997) Haemolymph protein in xanthidcrabs: its selective binding of saxitoxin and possible role in toxinbioaccumulation. Marine Biology. 128,599-606.

Longo, M. and Diaz, A (2013) Morphology of the claw closer muscle in two estuarine crab species(Crustacea,Varunidae): An ultrastructural study. Zoological Society ofJapan. 30(8), 663-669.

Peter K., Clark, P. (2000) The Indo-Pacific Pilumnidae XII. On thefamilial placement of Chlorodiella bidentata (Nobili, 1901) and Tanaochelesstenochilus Kropp, 1984 using adult and larval characters with theestablishment of a new subfamily, Tanaochelinae (Crustacea: Decapoda:Brachyura). Journal of Natural History.34(2), 207-245. 

Shaw, M., Tibbetts, I. (2004) Grazing by Metopograpsus Frontalis(Decapoda: Grasidea) on intertidal rock walls of Moreton bay. Proceedings of the royal society ofQueensland. 11, 95 – 101.

Shields, J. (2001) Ovicides julieae N. Gen., N.Sp. (Nemertea: Carcinonemertidae) on Xanthid Crabs from the Great Barrier Reef,Australia. Journal of CrustaceanBiology. 21(1), 304-312.

Sorokin, Y. (1995) Symbiosis in Reef Crustaceans. Coral Reef Ecology. 5, 191-193.

Stephensen, K., (1945) The Brachyura of the IranianGulf with an Appendix: The male pleopod of the Brachyura. Danish ScientificInvestigations in Iran, Part 4. Copenhagen, E. Munksgaard: 57-237, figs 1-60.

Swanson, B.(2013) Evolutionary variation in the mechanics of fiddler crab claws. BMC Evolutionary Biology. 13,137.